1. Field of the Invention
The present invention relates to a negative type planographic printing plate precursor and in particular to a negative type planographic printing plate precursor capable of highly sensitive writing with an infrared laser and excellent in handlability under white light.
2. Description of the Related Art
Conventionally, a PS plate having a lipophilic photosensitive resin layer provided on a hydrophilic substrate has been widely used as a planographic printing plate precursor, and a desired printing plate is obtained by a plate-making method which usually involves masked exposure (surface exposure) via a lithographic film and then removing non-image regions by dissolution. In recent years, digitalization techniques which involve electronic treatment, accumulation and output of image information with a computer are spreading. A wide variety of new image output systems compatible with the digitalization techniques came to be practically used. As a result, there has been demand for computer-to-plate (CTP) techniques of producing a printing plate directly by scanning a highly directional light such as a laser light according to digitalized image information without a lithographic film. It has been a critical technical issue to provide a planographic printing plate precursor suitable for these techniques.
As the planographic printing plate precursor capable of such scanning exposure to light, a planographic printing plate precursor comprising a hydrophilic substrate provided thereon with a lipophilic photosensitive resin layer (also referred to as the photosensitive layer) containing a photosensitive compound capable of generating active species such as radicals or Bronsted acid upon being exposed to laser light has been available. This planographic printing plate precursor is subjected to laser scanning according to digital information to generate active species acting on the photosensitive layer to cause physical or chemical change, thus making the layer insoluble, and then subjected to development to give a negative type planographic printing plate. In particular, a planographic printing plate precursor wherein a photopolymerizable photosensitive layer containing a photopolymerization initiator excellent in speed, an ethylenically unsaturated compound capable of addition polymerization, and a binder polymer soluble in an alkaline developing solution, and if necessary an oxygen-impermeable protective layer, are disposed on a hydrophilic substrate is superior in productivity, can be easily developed, and is excellent in resolution and inking property. Thus, it can be a planographic printing plate having desired printing performance.
Examples of the binder polymer included in the photosensitive layer used to date include organic polymers capable of alkali development, such as methacrylic acid copolymers, acrylic acid copolymers, itaconic acid copolymers, crotonic acid copolymers, maleic acid copolymers, and partially esterified maleic acid copolymers (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 59-44615, Japanese Patent Application Publication (JP-B) Nos. 54-34327, 58-12577, 54-25957, JP-A Nos. 54-92723, 59-53836, 59-71048 and 2002-40652). In conventional planographic printing plate precursors having a photosensitive layer containing such binder polymers, however, an image region, which is supposed to be cured, is not sufficiently cured and a developing solution permeates to the image region in the development step. As a result, the photosensitive layer is damaged, leading to deterioration in printing press life. To solve this problem, it has been common to carry out heat treatment after exposure preceding development of the image so as to promote curing.
The conventional planographic printing plate precursor comprising the UV-curing photosensitive layer undergoes undesired decomposition of an initiator with UV rays emitted from a fluorescent lamp etc. during storage, to cause a problem of staining in a non-image region. As a result, it has been unsuitable to store and handle the planographic printing plate precursor under white light. To solve this problem, for example JP-B No. 7-60268 and JP-A No. 8-137096 teach techniques in which coloring agents having a major absorption spectrum in the vicinity of safety light emission spectrum or compounds having an absorption spectrum peak in a range of 400 to 700 nm are contained in the protective layer. JP-A No. 8-248639 for example discloses an image-forming material provided with a protective layer whose reflection absorbance is set in a specific wavelength range. On the other hand, a planographic printing plate precursor for forming an image by being exposed to infrared laser light can provide a product excellent in handlability under white light (hereinafter referred to as safelight property) by selecting a UV-stable compound as a component in the photosensitive layer. However, some coloring agents commonly added, for the purpose of improving visibility of an image, to such image-forming materials to be exposed by an infrared laser have sub-absorption in the UV range. In such cases, the coloring agents absorb UV rays and thereby generate excited electrons. Thus, even in the image-forming materials to be exposed by infrared light, a polymerization initiator may initiate undesired polymerization due to excited electrons generated from the coloring agents when handled under white light. This may result a problem of staining in a non-image region, and therefore the stability of such image-forming materials under white light is very important in terms of practicality.
Therefore, there is demand for a planographic printing plate precursor which is excellent in safelight properties, can be handled under white light, and is excellent in printing press life even in a halftone dot region.